The present invention relates to highly delaminated hexagonal boron nitride powders, a process for making such powders, and the use of the resulting powders.
Several methods for milling boron nitride, in particular, hexagonal boron nitride (xe2x80x9ch-BNxe2x80x9d) are known in the art. One conventional process for milling h-BN is disclosed in Hagio et al., J. Am. Cer. Soc. 72:1482-84 (1989) (xe2x80x9cHagioxe2x80x9d). According to Hagio, a virgin h-BN powder (characterized by a particle size of about 10 xcexcm, a surface area of about 5 m2/g, and a thickness of about 100 nm) is milled by grinding with tungsten carbide mortar (WC) in air. The apparent purpose of Hagio""s milling operation is to increase the surface area of the h-BN powder, thereby increasing its reactivity. When milled in this manner for 24 hours, the resultant h-BN powder has a lower particle diameter (2 xcexcm), a higher surface area (54 m2/g), and is slightly thinner (71 nm). The data reported by Hagio suggests that the final geometry of the milled powder is not dependent upon the starting powder purity. Although Hagio reports a reduction in the platelet thickness, Hagio""s milling operation primarily results in BN particle fracture, thereby reducing the particle diameter, resulting in an increased surface area.
In U.S. Pat. No. 5,063,184 to Tsuyoshi et al. (xe2x80x9cTsuyoshixe2x80x9d), it is reported that high surface area, highly reactive h-BN powders are useful in providing high density, pressureless sintered h-BN components. In each example in Tsuyoshi, the virgin h-BN is milled in either air or nitrogen.
The present invention is directed towards providing an improved milling method for producing h-BN powders.
The present invention relates to a powder comprising hexagonal boron nitride particles having an aspect ratio of from about 50 to about 300.
The present invention also relates to a method of making delaminated hexagonal boron nitride powder. This method involves providing hexagonal boron nitride powder and milling the hexagonal boron nitride powder in a mixture under conditions effective to produce delaminated hexagonal boron nitride powder having an aspect ratio of from about 50 to about 300.
The method of the present invention produces more highly delaminated, high aspect ratio boron nitride powder. Whereas the dry milling procedures of the prior art increase the surface area of the BN particle essentially by particle fracture (i.e., by reducing the particle diameter), the method of the present invention provides similar increases in surface area but does so by particle delamination (i.e., by reducing particle thickness). The resulting boron nitride powder has a high aspect ratio (a large particle diameter and a small particle thickness) which is useful in certain applications, e.g. as a processing aid for the extrusion of polymers. In particular, the delaminated BN powders of the present invention are more effective at lowering the die wall/polymer interfacial friction during extrusion, leading to a decrease in extrusion pressures and delaying further the onset of gross melt fracture to higher effective shear rates.